1. Field
Apparatuses and methods consistent with exemplary embodiments relate to magnetic resonance imaging (MRI), and, more particularly, to T1 tissue characterization using a multi-slice imaging.
2. Related Art
When a substance such as human tissue is subjected to a uniform magnetic field, i.e., a static magnetic field B0, the individual magnetic moments of the nuclear spins in the tissue attempt to align with the static magnetic field B0, but precess about it in random order at their characteristic Larmor frequency. A net magnetization moment Mz is generated in the direction of the static magnetic field B0, but the randomly oriented magnetic components in the perpendicular plane, i.e., transverse x-y plane, cancel one another. If, however, the substance is subjected to a magnetic excitation field B1 which is in the x-y plane and which is near the Larmor frequency, the net magnetization aligned moment Mz may be rotated, i.e., tipped, into the x-y plane to generate a net transverse magnetic moment Mt, which is spinning in the x-y plane at the Larmor frequency. An MR signal is emitted by the excited nuclei, i.e., spins, after the excitation magnetic field B1 is terminated, and the MR signal may be received by a radio-frequency (RF) coil and processed to form an image.
In MRI systems, the amplitude of the MR signal is dependent on the spin-lattice relaxation process that is characterized by the time constant T1, i.e., a spin-lattice relaxation time. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization, i.e., z-magnetization.
Advances in cardiac MR (CMR) allow for the non-invasive imaging of interstitial diffuse fibrosis using quantitative T1 mapping. In this technique, the voxel-wise calculation of the longitudinal magnetization recovery time provides spatially-resolved quantitative characterization of the myocardial tissue composition. The myocardial T1 times vary between various cardiomyopathies, and both native, i.e., non-contrast, and post-contrast myocardial T1 times have been used to evaluate patients with various cardiomyopathies. Furthermore, extra-cellular volume (ECV) may be calculated by measuring native and post-contrast T1, taking into account the patient hematocrit.
Lately, various imaging pulse sequences have been proposed for myocardial T1 mapping, as for example, the modified look-locker inversion recovery (MOLLI) pulse sequence. However, to overcome the problem of time-consuming long rest periods between the inversion pulses, MOLLI samples the longitudinal magnetization recovery curve multiple times after a single magnetization preparation pulse, which hinders the accuracy. Thus, MOLLI suffers from inaccurate T1 estimates due to the heart rate, sensitivity to T2 times, and magnetization transfer dependencies.
To reduce the scan time and eliminate heart rate variability, variations of the MOLLI pulse sequence have been proposed, as for example, the 5(3)3 MOLLI or the shortened MOLLI (ShMOLLI). However, these sequences still suffer from inaccurate measurements leading to underestimated T1 of a healthy myocardium by up to approximately 30 percent.
Therefore, there is a need for an accurate and precise T1 mapping imaging sequence.
Additionally, myocardial T1 mapping is frequently performed using a two-dimensional (2D) sequence during breath-holds, with the acquisition of a single breath-hold per slice. Many methods have been proposed which use a single mid left ventricular (LV) slice for the calculation of T1 maps, and report a single T1 time for each patient. However, a single value might not characterize the regional myocardial tissue composition over the entire ventricle. While in some myopathies, such as amyloidosis, there might not be much variation across the myocardium, in many other cardiomyopathies, such as hypertrophic cardiomyopathies, there may be regional variations that could directly impact T1 measurements. Therefore, full LV coverage is needed for accurate characterization of the LV myocardium.
Currently, multiple separate 2D scans have to be performed for different slices to obtain a complete LV coverage in clinical practice. This requires numerous breath-holds, which inconvenience the patients.
Thus, methods and apparatuses are needed for improved T1 mapping that meets the needs of clinical applications using clinically-available resources and improves scan times and convenience of patients.